1. Field of the Invention
The present invention relates to a CO2 recovering apparatus for reducing an amount of O2 accompanying oxygen absorbing liquid, and reducing the concentration of oxygen contained in the CO2 gas regenerated in a regenerator.
2. Description of the Related Art
It has come to be pointed out that one of the causes of the global warming is a greenhouse effect of CO2, and it has became an urgent task, also internationally, to provide a countermeasure for CO2 to protect the global environment against the warming. CO2 is generated by any human activities combusting fossil fuels, and there are increasing demands for suppressing CO2 emissions. Along with such an increasing demand, researchers are energetically investigating a method for reducing and recovering CO2 included in flue gas, to apply in a power plant that consumes a large amount of fossil fuels, such as a thermal plant. In such a method, flue gas emitted from a steam generator is brought into contact with an amine-based CO2 absorbing liquid to allow such absorbing liquid to absorb the CO2, and the recovered CO2 is stored therein without being released into the air. As processes for reducing and recovering CO2 from the flue gas using the CO2 absorbing liquid, Japanese Patent Application Laid-open No. H3-193116, for example, brings flue gas into contact with the CO2 absorbing liquid in an absorber, heats an absorbing liquid that has absorbed CO2 in a regenerator, isolates CO2 as well as regenerates the absorbing liquid, and circulates the absorbing liquid back to the absorber and reuses the absorbing liquid therein.
As shown in FIG. 2, a conventional CO2 recovering apparatus 100 as mentioned above includes a flue gas cooler 14, a CO2 absorber 16, and a regenerator 18. The flue gas cooler 14 cools flue gas 12 containing CO2 and O2 emitted from an industrial combustion facility 11, such as a steam generator or a gas turbine, with cooling water 13. The CO2 absorber 16 further includes a CO2 recovering unit 16A. The CO2 recovering unit 16A brings the flue gas 12, containing the cooled CO2, into contact with CO2 absorbing liquid (hereinafter, also referred to as “absorbing liquid”) 15 that absorbs CO2, to reduce CO2 in the flue gas 12. The regenerator 18 causes CO2 absorbing liquid (hereinafter, also referred to as “rich solvent”) 17 that has absorbed CO2 to release CO2 to regenerate the CO2 absorbing liquid.
In the CO2 recovering apparatus 100, the regenerated CO2 absorbing liquid (hereinafter, also referred to as “lean solvent”) 15 having CO2 reduced in the regenerator 18 is reused in the CO2 absorber 16 as the CO2 absorbing liquid.
By a CO2 recovering method using the CO2 recovering apparatus 100, a flue gas booster fan 20 raises the pressure of the flue gas 12 emitted from an industrial combustion facility such as a steam generator or a gas turbine and containing CO2. The flue gas 12 is then sent into the flue gas cooler 14, cooled by way of the cooling water 13, and then sent into the CO2 absorber 16.
The CO2 absorber 16 then brings the flue gas 12 in a counter-current contact with the CO2 absorbing liquid 15 that is based on amine-based solvent, allowing the CO2 absorbing liquid 15 to absorb the CO2 contained in the flue gas 12 by way of chemical reaction (R—NH2+H2O+CO2→R—NH3HCO3).
A washing unit 16B, included in the CO2 absorber 16, brings the flue gas having CO2 reduced in the CO2 recovering unit 16A into a gas-liquid contact with circulating condensate water 19. The condensate water 19 contains the CO2 absorbing liquid, and is supplied via a nozzle included in a washing unit 16B. In this manner, the CO2 absorbing liquid 15 that has accompanied the flue gas having CO2 reduced is recovered. Flue gas 12 having CO2 reduced is released out of the system.
A rich solvent pump 22 increases the pressure of the rich solvent that is the CO2 absorbing liquid 17 that has absorbed CO2. Then, a rich/lean solvent heat exchanger 23 heats the rich solvent by way of the CO2 absorbing liquid 15 that is lean solvent regenerated by the regenerator 18, and supplied into the regenerator 18.
The rich solvent discharged into the regenerator 18 through the top thereof causes an exothermal reaction, thus releasing a majority of CO2. The CO2 absorbing liquid that has released some or a majority of CO2 in the regenerator 18 is called semi-lean solvent. By the time the semi-lean solvent reaches the bottom of the regenerator 18, almost all of the CO2 is removed, turning the semi-lean solvent into the absorbing liquid 15. A regenerating heater 24 then heats the lean solvent by way of steam 25, supplying steam inside the regenerator 18.
CO2 gas 26 is guided out from the top of the regenerator 18, together with the steam that has been released from the rich solvent and semi-lean solvent in the regenerator 18. A condenser 27 then condenses steam contained in the CO2 gas 26, and a separation drum 28 separates water from the CO2 gas 26. The CO2 gas 26 is then released out of the system, and recovered separately. The recovered CO2 gas 26 is injected into an oilfield using enhanced oil recovery (EOR) method, or stored in an aquifer as a countermeasure for global warming.
The water separated in the separation drum 28 is pumped up to the top of the regenerator 18 by way of a condensed-water circulating pump 29. The rich/lean solvent heat exchanger 23 cools the regenerated CO2 absorbing liquid (lean solvent) 15 by way of the rich solvent 17. A lean solvent pump 30 then increases the pressure of the lean solvent 15. After being cooled down by a lean solvent cooler 31, the lean solvent 15 is supplied into the CO2 absorber 16.
In FIG. 2, the reference numeral 11a denotes to a flue for the flue gas 12; the reference numeral 11b denotes to a stack; and the reference numeral 32 denotes to steam-condensed water. The CO2 recovering apparatus may be either added to an existing flue gas source to recover CO2, or installed with a flue gas source that is to be newly installed. A door that can be opened and closed is attached on the stack 11b. The door is closed while the CO2 recovering apparatus is operating, and opened while the flue gas source is operating but the CO2 recovering apparatus is not operating.
When the flue gas 12 containing CO2 is brought into contact with CO2 absorbing liquid 15 in the CO2 absorber 16, air bubbles can get caught in the absorbing liquid that has flowed down in the CO2 absorber at the bottom thereof, and the rich solvent 17 is stored with the bubbles being caught.
The air bubble contains oxygen (O2) that is also a constituent of the flue gas 12. Although the concentration of oxygen dissolved in the absorbing liquid is generally approximately 25 parts per million, the concentration of the oxygen in the air bubbles being caught in the absorbing liquid could reach approximately 100 parts per million.
Upon transporting CO2 containing oxygen via a pipeline, if CO2 containing H2S, supplied from another CO2 source, is mixed therewith, H2S may react to oxygen, resulting in deposit of solid sulfur (S). Therefore, there is a demand for reducing oxygen (O2) as much as possible.
Furthermore, upon injecting CO2 into an oilfield using EOR, oxygen could cause facility corrosion. Moreover, if oxygen is mixed to CO2 when the recovered CO2 gas is used for chemical applications, a catalyst may get deteriorated. Therefore, it is desired to reduce oxygen from CO2 as much as possible.
Published Japanese Translation of PCT Application No. 2004-524147, for example, suggests using a flash tank or a vacuum pump to reduce oxygen in absorbing liquid.
However, in the oxygen reducing technology according to the Published Japanese Translation of PCT Application No. 2004-524147, a separate reducing apparatus such as a flash tank or a vacuum pump is required, thus further requiring facility cost and reduction energy, resulting in a cost increase.